Use weak cell in LoadGlobal handler.

BUG=v8:3629
LOG=N

Review URL: https://codereview.chromium.org/816923004

Cr-Commit-Position: refs/heads/master@{#25902}
Follow-on to hydrogen fix keyed loads with string keys for vector ics.

r25817 optimized hydrogen keyed loads with string keys.
(https://codereview.chromium.org/755513003).
This CL adapts that work for the vector-ic keyed load.

BUG=

Review URL: https://codereview.chromium.org/820673002

Cr-Commit-Position: refs/heads/master@{#25903}
[turbofan] false is also falsish.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/814833006

Cr-Commit-Position: refs/heads/master@{#25904}
Use weak cell to embed map in CompareNil IC.

BUG=v8:3629
LOG=N

Review URL: https://codereview.chromium.org/812353002

Cr-Commit-Position: refs/heads/master@{#25905}
MIPS64: Use weak cell in LoadGlobal handler.

Port bc0d2e237b1b624bed26ba849529b40329ea72a2

BUG=v8:3629
LOG=N
TBR=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/814193004

Cr-Commit-Position: refs/heads/master@{#25906}